During current content Disc is pulling out as far as throughput, the changes with the upcoming Patch might change that.
But as posted above the spec you are more comfortable playing as will most likely give you better numbers.

All healing specs can do the job of healing raids. If you just need to step up and heal, any spec can do the job.
For this reason I would recommend playing what you enjoy rather than playing what is the flavour of the month.
Try both specs and see which spec suits you the most!

Disc requires you to plan ahead to mitigate damage before it hits. But right now it can currently be played by mostly spamming one button, win healing meters and make healing look easy.
Holy is more reactive and relies on using multiple different heals to excel. It's IMO a lot more fun too. But chakra is limiting what you can do well, and mana is undeniably tighter.

Disc is currently better than holy in virtually all aspects of the game, though that really only matter if you plan on doing high end raiding. There is one fight that seriously favor holy (tsulong), there is a handful that favor disc, and the rest is really up for either.

I do not know how this will play out in 5.2 though. This may and probably will change in the next patch, and it is too early to really tell. I would assume Holy ends up with doing more slightly HPS and disc ends up with slightly better damage mitigation cooldowns, though this is quite a sore topic around these parts and I'd rather not start that debate up.

Disc currently can outpace Holy. However if you are programmed into a reactive healing style, than you will perform better at Holy than Disc.

This isn't exactly true. There are some proactive tactics to healing as Holy, including pre-casting, pre-stacking Serendipity and refreshing Renew using Cascade right before damage (particularly on Garalon's Crush). Also, if mana allows it, you can use Holy's Mastery to stack a rolling HoT on a target (say, a tank about to take damage) in much the same way a Disc priest stacks DA prior to damage. In 5.2 it will get a bit harder for Disc priests to pre-shield outside of SS and PWS, and any damage that gets through will see Disc priests less effective at healing than Holy. Since there are usually fights that require raw healing in every other tier or have a mechanic that absorbs don't work with, Disc priests in general do tend to have to keep a Holy off-spec "in case of emergency", whereas Holy Priests are safe to run with a Shadow off-spec.

This isn't exactly true. There are some proactive tactics to healing as Holy, including pre-casting, pre-stacking Serendipity and refreshing Renew using Cascade right before damage (particularly on Garalon's Crush). Also, if mana allows it, you can use Holy's Mastery to stack a rolling HoT on a target (say, a tank about to take damage) in much the same way a Disc priest stacks DA prior to damage. In 5.2 it will get a bit harder for Disc priests to pre-shield outside of SS and PWS, and any damage that gets through will see Disc priests less effective at healing than Holy. Since there are usually fights that require raw healing in every other tier or have a mechanic that absorbs don't work with, Disc priests in general do tend to have to keep a Holy off-spec "in case of emergency", whereas Holy Priests are safe to run with a Shadow off-spec.

That's assuming you are on Serenity Chakra. I doubt many holy priests go Serenity for this fight (Garalon), as they lose a lot of AoE power needed for his fight, such as CoH and PoH. Although you could be right, I haven't checked logs so I'm just saying what seems logical to me. In Sanctuary chakra you also use PoH a lot more, giving you plenty of DI procs which help a lot, especially on this fight.